The letter comes as politicians debate a home affairs select committee report that warns that government action is needed "now, more than ever" to consider all the alternatives for drug policy.

It reads: "Worldwide there is growing recognition that the 'war on drugs' has failed – having cost billions of dollars and caused tens of thousands of deaths.

"In the UK, scientists, politicians, lawyers and police increasingly agree that we need to review existing policy, which costs taxpayers £3 billion a year but does little to address the root causes of addiction and pointlessly criminalises people.

"By agreeing to an independent review to determine whether the Misuse of Drugs Act 1971 is effective or good value for money, the Government can prove its willingness to acknowledge failures within existing policy – and join in the global effort towards an alternative strategy based on evidence."